Brake bands which surround a brake drum are generally known as braking devices. The brake bands are made of an resilient material such as spring steel or the like whereby a spring return force is available after the brake band has been applied to the brake drum. This return force is utilized for returning the brake band to its initial position. However, consideration should be given to the coefficient of friction when selecting the resilient material since this determines the braking times.
Braking devices comprising brake bands and brake drums are subjected to the highest stress when used in motor-driven chain saws. The braking device operates to immediately bring the saw chain to standstill within the shortest time (a fraction of a second), for example, to prevent a continued running of the saw chain under all circumstances such as when the chain saw is placed down after use so that others are not endangered thereby. For increasing safety, the throttle lever latch is coupled with a braking device acting on the saw chain. By depressing the throttle lever latch, the braking device is disengaged and when releasing the gas throttle latch, the braking device is applied and the saw chain is brought to standstill. With this continued braking of the saw chain, the brake band is subjected to the highest stress which quickly leads to wear and a lower service life.
With motor-driven chain saws, it is a problematical condition that the braking device, which is often mounted directly at the driving two-stroke engine, often comes into contact with oil which penetrates between the flexible brake band and the brake drum and leads to a reduction in the coefficient of friction and thereby to increased braking times.